1. Field of the Invention
The present invention relates to a method of fabricating novel single-crystal substrates of .alpha.-silicon carbide (.alpha.-SiC).
2. Description of the Prior Art
Silicon carbide is a semiconductor material having wide forbidden band gaps (2.2 to 3.3 eV) and exhibiting very stable properties thermally, chemically and mechanically and has the feature of being highly resistant to damage due to radiation. The material has both conductivities of the p-type and n-type, whereas this is seldom the case with semiconductors having wide forbidden band gaps. Accordingly silicon carbide appears useful as a semiconductor material for light-emitting or photodetector devices for visible lights of short wavelengths, for electronic devices operable at high temperatures or with high electric power, for highly reliable semiconductor devices and for radiation-resistant devices. Further silicon carbide will provide electronic devices which are usable in an environment where difficulties are encountered with devices made of conventional semiconductor materials, thus greatly enlarging the range of applications for semiconductor devices. Other semiconductor materials such as semiconductor compounds of elements from Groups II and VI or from Groups III and V generally contain a heavy metal as the main component and therefore have the problems of pollution and resources, whereas silicon carbide is free of these problems and accordingly appears to be a promising electronic material.
There are many crystal structures of silicon carbide (called "polytype") which are generally divided into the .alpha.-type and .beta.-type. Silicon carbide of the .beta.-type has a cubic crystal structure and is the smallest in forbidden band gaps (2.2 eV) of all forms of silicon carbide, while .alpha.-silicon carbide is of hexagonal or rhombohedral crystal structure and has relatively large forbidden band gaps of 2.9 to 3.3 eV. Because of the large forbidden band gaps, .alpha.-silicon carbide is expected to be a promising semiconductor material for optoelectronic devices, such as light-emitting devices and photodectectors, for use with blue and other visible lights of short wavelengths and near-ultraviolet rays. Although zinc sulfide (ZnS), zinc selenide (ZnSe), gallium nitride (GaN), etc. are materials which appear useful for light-emitting devices for blue or other visible lights of short wavelengths, the crystals of these materials usually available have conductivity of only one type, i.e. p-type or n-type, and difficulties are encountered in obtaining crystals having conductivity of both types. In contrast, .alpha.-silicon carbide readily provides a crystal of both p-type and n-type conductivities to afford a p-n junction. It is therefore expected that the material will realize light-emitting devices and photodetectors having outstanding optical characteristics or electrical characteristics. Further because of the exceedingly high stability in its thermal, chemical and mechanical properties, the material will be usable for wider applications than the other semiconductor materials.
Despite these many advantages and capabilities, .alpha.-silicon carbide has not been placed into actual use because the technique still remains to be established for growing .alpha.-silicon carbide crystals as controlled in size, shape and quality with good reproducibility, as required for the commercial mass production of silicon carbide substrates of large area with high quality and high productivity.
Conventional processes for preparing .alpha.-silicon carbide single-crystal substrates on a laboratory scale include the so-called sublimation method [also termed the "Lely method"; "Growth Phenomena in Silicon Carbide", W. F. Knippenberg: Phillips Research Reports, Vol. 18, No. 3, pp. 161-274 (1963). (Chapter 8, "The Growth of SiC by Recrystallization and Sublimation", pp. 244-266)] wherein silicon carbide powder is sublimed in a graphite crucible at 2,200.degree. C. to 2,600.degree. C. and recrystallized to obtain a silicon carbide substrate, the so-called liquid-phase method ["Growth of Silicon Carbide from Solution" R. C. Marshall: Material Research Bulletin, Vol. 4, pp. S73-S84 (1969)] wherein silicon or a mixture of silicon with iron, cobalt, platinum or like impurities is melted in a graphite crucible to obtain a silicon carbide substrate, and the Acheson method "Growth Phenomena in Silicon Carbide" W. F. Knippenberg: Philips Research Reports, Vol. 18, No. 3, pp. 161-274 (1963). (Chapter 2 "Preparative Procedures", pp. 171-179)] which is generally used for commercially producing abrasives and by which a silicon carbide substrate is obtained incidentally. Blue light-emitting diodes are fabricated using a substrate of .alpha.-silicon carbide obtained by such a crystal growth method, by forming on the substrate a single-crystal layer of .alpha.-silicon carbide by liquid-phase epitaxial growth (LPE) or chemical vapor deposition (CVD) to provide a p-n junction.
However, although the sublimation method or the liquid-phase method affords a large number of small single crystals, it is difficult to prepare large single-crystal substrates of good quality by these methods since many crystal nuclei occur in the initial stage of crystal growth. The silicon carbide substrate incidentally obtained by the Acheson method still remains to be improved in purity and crystal quality for use as a semiconductor material, while large substrates, if available, are obtained only incidentally. Thus, the conventional crystal growth methods for preparing substrates of .alpha.-silicon carbide have difficulties in controlling the size, shape, quality, impurities, etc. and are not suited to the commercial production of single-crystal substrates of silicon carbide in view of productivity. Although light-emitting diodes are produced by preparing substrates of .alpha.-silicon carbide by the conventional method and subjecting the substrates to liquid-phase epitaxy or chemical vapor deposition as already mentioned, no progress has been made in commercial mass production since there is no method of industrially preparing .alpha.-type single-crystal substrates having a large area and a high quality.
On the other hand, it is possible to epitaxially grow a single-crystal film of silicon carbide by CVD, LPE, molecular beam epitaxy (MBE) or like process on a single-crystal substrate of silicon (Si), sapphire (Al.sub.2 O.sub.3), .beta.-silicon carbide (.beta.-SiC) or the like which differs from .alpha.-silicon carbide in component element or crystal structure, whereas the silicon carbide films obtained by this method are only of the .beta.-type having a cubic crystal structure.